Welcome to Trisquel! To answer two of your questions:
No, chances are you won't have 3D acceleration with fully libre drivers unless you're using something like a GeForce 8400.
Also, the best way (in my opinion) for window snapping as you described it is to install the XFCE desktop environment. In the settings menu you can find a box labeled "switch desktops when dragging to the edge of the screen" or something like that. Simply uncheck that box and viola: dragging the window to the edge of the screen snaps it to half-screen view.
Also if you feel like learning an entirely new window manager, you could try a tiling manager like "Awesome".
